As for a motor drive that drives and controls a motor such as a brushless motor, if it detects an abnormal current, it generally stops power supply to the motor to protect the circuit. On the other hand, at the rotor reversal of the motor or when the motor is rotated in the direction opposite to the driving direction by an external load, a large current flows through a motor with low resistance. Thus, if the motor drive detects the large current as an abnormal current and stops power supply, the rotor is placed in an idle running state in which it rotates freely, and its reversal is delayed. Accordingly, a motor drive with a vector control system carries out current feedback based on phase currents, and appropriately controls regenerative currents at the reversal.
On the other hand, a motor drive without the current feedback requires such control as suppressing an overcurrent at the reversal. As prior art for such a purpose, Patent Documents 1-4 are known, for example.
The Patent Document 1 relates to a technique that protects from an overcurrent using a microprocessor. It devises an abnormal countermeasure if a result of subtracting the product of a motor rotational speed detection value and a counter electromotive force coefficient from a motor applied voltage command value exceeds the product of a prescribed overcurrent value and an armature resistance.
In addition, the Patent Document 2 limits a voltage command value for driving a motor if the motor driving current exceeds a prescribed value.
Furthermore, the Patent Document 3, which controls the voltage command value through the feedback of the motor rotational speed, limits the voltage command value in such a manner that it operates at a value lower than a motor induced voltage estimated from the rotational speed, that it produces necessary torque, and that it does not bring about an overcurrent.
In addition, the Patent Document 4 limits the voltage command value by using as a limit voltage the sum of an induced voltage estimation value, which consists of the product of a speed command that causes a motor to make a soft start and an induced voltage constant, and an impedance voltage drop estimation value, which consists of the product of the motor impedance and the maximum current command.